Though a two-component developing method has been used widely as an excellent developing method, in an electrophotographic developing method, it still has some problems even today. Among the problems, there is a strong demand for a two-component developer which can be used for a long time and their properties are constant and stabilized for their total service period.
The electrification of toner is carried out by the friction of the toner particle with the resin coated surface of the carrier. One of the problems, in particular, is that the resin-coated layer on a carrier surface in the developer is worn away and peels off after developing operations are repeated for a long time, or, a constituent of the toner sticks and coheres on the carrier surface, resulting in deterioration of the ability of the carrier to electrify the toner. In the actual developing, however, developing operations are continued under the condition where potential on a developing electrode is constant, whereby a toner-covering rate on the carrier surface in the developer is gradually reduced. Therefore, image density may be fluctuated especially in a full-color image, partial missing of an image may occur on the rear edge of a solid image caused by a relative velocity difference between a developing sleeve and a photoreceptor, or, when a halftone portion and a solid image exist together, partial missing of an image may also occur on the rear edge of a halftone image in the boundary portion between the leading edge of the solid image and a halftone portion is generated.
With the aforesaid background, there has been proposed a developing system (so-called trickle developing system, see Patent Document 1) in which, when toner is refilled corresponding to an amount consumed in the development, carrier is also added to gradually replace the carrier in the developing device, whereby the variation of the charge accumulated on the toner is suppressed and thus the developing density is stabilized.
On the other hand, in recent electrophotography, a toner having a smaller particle diameter has been widely studied in order to attain high image quality. However, there have been problems of decreased fluidity of toner particles and lack of stability in charging property of the toner, where the toner is charged due to the friction with the carrier particles. As a result, there is a tendency that the amount of added external additive is increased in order to assure the fluidity of the toner. For that reason, the amount of the transferred external additive to the carrier particles is increased while the developer is stored or is subjected to development. Accordingly, the ability of the carrier to elecrify the toner is lowered, and the difference in developing characteristics between the initial stage of developer manufacturing and after the lapse of time becomes greater. It has been often difficult to obtaining stable image quality for a long time.
In the aforesaid trickle developing method, the fluctuation of the amount of charge on the toner particles has been tried to be suppressed by gradually replacing the carrier with a fresh carrier. However, when the diameter of the toner is small, the amount of charge on the toner particles sometimes becomes smaller. This reason is considered as follow; in the case of a small diameter toner wherein the surface area per a unit volume is large, the amount of external additive to be added is large, and thereby, the amount of the external additive transferred to the newly supplied carrier is also large, resulting in the decrease in the amount of charge on the toner particles.
Specifically, when print of many sheets consuming a large amount of toner are carried out, the amounts of toner and carrier to be supplied become greater, and the amount of charge on the toner particle tends to decrease due to the transfer of the external additive to the newly supplied carrier particles, which may cause undesired image fog or unevenness in image density.
Patent Document 1 Examined Japanese Patent Application Publication No. 2-21591